CSS 650 Advanced Plant Breeding

Module 3:Changes in gene frequency due to selection

Systematic changes in gene frequency

Falconer & Mackay, Chapter 2

• Predictable in both magnitude and direction

– Migration

– Mutation

– Selection

• Quantities of interest

– Δq = q1 – q0

– qt

– q at equilibrium (when Δq = 0)

Selection

Individuals differ in their contribution of genes to the next generation fitness = adaptive value = selective value

Types of selection

• Viability - the probability of survival through each reproductive stage– Darwin’s natural selection

• Fertility – number of offspring produced– may work in opposition to survival

– Darwin’s sexual selection

Components of fitness

Zygote

Adult

Gamete

Zygote

viability

mating successfertility (capability to have offspring)fecundity (#gametes)

viability(survival)

Artificial selection

• fitness is defined in the context of the environment in which individuals live, mate, and reproduce

Relative fitness

• Absolute fitness refers to the number of gametes transmitted by a zygote, but this is seldom observed directly

• We generally make observations on zygotes, so we measure relative fitness in comparison to either the average for the population or the best genotype

Genotype A1A1 A1A2 A2A2

Fitness 1W

W

11

11 hs1W

W

11

12 s1W

W

11

22

s = coefficient of selection

Selection

• To quantify fitness, we have to consider the degree of dominance with respect to fitness – may not be the same as the dominance with

respect to the expression of the primary trait controlled by the gene

Degree of Dominance for Fitness

no dominance

partial dominance

complete dominance

overdominance

A1A1A2A2 A1A2

A1A1A2A2 A1A2

1-s 1-(1/2)s 1

1-s 1-hs 1

A1A1A2A2

A1A2

1-s 1

A1A1A2A2 A1A2

1-s2 1-s1 1

fitness

Selection against a recessive gene

Genotype A1A1 A1A2 A2A2 Total Initial frequencies p2 2pq q2 1 Fitness 1 1 1-s From gametes p2 2pq q2(1-s) 1-sq2

2

2

2

2

2

2

1 1111

11

sqsqq

sqqqsq

sqpqsq

q

2 2

1 2 2

(1 )

1 1

q sq sq qq q q q

sq sq

Genetic load = proportion of the population that does not reproduce due to presence of a gene i.e., the reduction in fitness

Selection against a recessive lethal - example

• What will the frequency of the recessive allele be after t generations?

2 20

1 2 20

(1 )

1 1 (1 )(1 ) 1

qq sq q q q qq

sq q q q q

s=1

0

01t

qq

qt

0

1 1

t

tq q

to go from q=0.20 0.011 1

1050.01 0.20

t generations

What could a breeder do to speed up the process?

Change in gene frequency with selection

Falconer & Mackay, pg 28

no dominance

partial dominance

complete dominance of A1

selection against A2

overdominance

complete dominance of A1

selection against A1

Change of gene frequency due to selection

Complete Dominances=0.20

q(-)=selection against A2

(+)=selection for A2

Falconer & Mackay, pg 30

Direction of selection

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html#Fitness